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<h2 align=center>stdarg</h2>
<h4 align=center>OS/161 Reference Manual</h4>

<h3>Name</h3>
<p>
stdarg - handle functions with variable arguments
</p>

<h3>Library</h3>
<p>
Standard C Library (libc, -lc)
</p>

<h3>Synopsis</h3>
<p>
<tt>#include &lt;stdarg.h&gt;</tt><br>
<br>
<tt>va_start(va_list </tt><em>ap</em><tt>,
</tt><em>start-argument</em><tt>);</tt><br>
<br>
<tt>va_end(va_list </tt><em>ap</em><tt>);</tt><br>
<br>
<em>type</em><br>
<tt>va_arg(va_list </tt><em>ap</em><tt>, </tt><em>type</em><tt>);</tt><br>
<br>
<tt>va_copy(va_list </tt><em>dest</em><tt>,
va_list </tt><em>src</em><tt>);</tt><br>
</p>

<h3>Description</h3>
<p>
Functions where the number of arguments is not fixed at compile time
can be written using the stdarg facility. This provides a type,
<tt>va_list</tt>, and the macros listed above. These allow iterating
through the arguments.
</p>

<p>
<tt>va_start</tt> initializes a <tt>va_list</tt> <em>ap</em> to point
to the current function's arguments. The <em>start-argument</em>
argument should be the name of the last fixed parameter in the calling
sequence.
(There must be at least one fixed parameter.)
</p>

<p>
<tt>va_end</tt> cleans up a <tt>va_list</tt> once it is no longer
needed. While failure to use <tt>va_end</tt> may have no effect on
some architectures (in fact, in some cases <tt>va_end</tt> does
nothing at all) on other architectures it may be fatal.
</p>

<p>
<tt>va_arg</tt> retrieves the next argument, which is presumed to be
of type <em>type</em>. The function must have some way to determine
what types to expect, and how many arguments, as this information
cannot be extracted from the argument list itself. To rewind, use
<tt>va_end</tt> and then <tt>va_start</tt> again.
</p>

<p>
Remember that default C argument promotions occur when passing the
variable arguments. There is no run-time checking of any kind, and
little to no compile-time checking: if you use <tt>va_arg</tt> to
retrieve a type different from that which was passed, you will
silently get garbage for that and (usually) all subsequent arguments.
</p>

<p>
<tt>va_copy</tt> assigns a copy of <em>src</em> to
<em>dest</em>. Subsequent operations on either will not affect the
other. Both copies need to be cleaned up with <tt>va_end</tt>.
</p>

<h3>Restrictions</h3>
<p>
Because the <tt>va_list</tt> is not necessarily a simple type, but may
involve pointers to state maintained elsewhere, it is not necessarily
a simple value. Thus, assigning <tt>va_list</tt> objects to each other
with `=', memcpy, or the like, or passing them to functions, may not
give multiple independent objects. When in doubt, use
<tt>va_copy</tt>, or invoke <tt>va_start</tt> multiple times.
</p>

<h3>Return Values</h3>
<p>
<tt>va_start</tt>, <tt>va_end</tt>, and <tt>va_copy</tt> do not return
anything. <tt>va_arg</tt> returns the value of the requested argument.
</p>

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